Rails 4 contains lots of updates to the web framework and these free videos from Code School should get you up to speed in no time. Take a look, and if you want additional practice using these features you can find a bunch of in-browser challenges over on Code School's official Rails 4: Zombie Outlaws course.

Warning: These videos will only make sense if you already know Rails. If you're looking for an introduction head over to Rails for Zombies.